Ursa Major, the Great Bear

Plus the planets for May.

One of the most well-known constellations in our sky is Ursa Major, the Great Bear. On May nights if you face north and look about two-thirds of the way up in the sky, you can find Ursa Major hanging upside-down facing eastward. The Great Bear is often identified by locating the asterism (informal group of stars) known as the Big Dipper.

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High in our northern sky, Ursa Major hangs upside-down. The hindquarters of the Great Bear is the Big Dipper, a well-known but informal grouping of stars. The two stars at the end of the Big Dipper point to Polaris, the Pole Star. All the stars in the sky rotate around the pole once a day. This takes all the stars of the Big Dipper below our northern horizon except for Dubhe, the northern pointer star.

The Big Dipper is the hindquarters of Ursa Major and is composed of seven bright stars, Alkaid, Mizar, Alioth, Megrez, Phecda, Dubhe and Merak. Dubhe and Merak are at the east end of the Dipper and are the pointer stars. If you follow the line formed by these two stars, you will find Polaris and the North Celestial Pole, which is the extension of the Earth's axis of rotation into the sky. All the stars in the sky appear to rotate around this point once every 24 hours. The middle star of the handle of the Dipper, Mizar, is actually a multiple star. Mizar is magnitude 2.1 and Native Americans would use it as a vision test. If the youngster could see the brighter companion, Alcor, at magnitude 4.0 just 12 minutes-of-arc away, they were judged to have good vision.

The Great Bear comes from Greek mythology, in which Callisto was a beautiful nymph whom Zeus lusted after. To keep her husband from Callisto, Hera changed Callisto into a bear. Later, Callisto's son, Arcas, came across Callisto in her bear form and prepared to shoot her with an arrow. To prevent this tragedy, Zeus turned Arcas into a bear as well and put them both into the sky — Callisto as Ursa Major and Arcas as Ursa Minor. Many other cultures have considered this grouping to be a bear, including Jewish and Iroquois.

Ursa Major is the third-largest constellation in the sky, so it contains many deep-sky objects. Most are galaxies, including the Pinwheel (M 101), Bode's "Nebula" (M 81), and the Cigar Galaxy (M 82). M 82 is magnitude 8.4 and is an irregular galaxy that is 11.2 minutes-of-arc long and 4.3 minutes-of-arc wide. This galaxy is undergoing a starburst, the rapid and widespread formation of stars caused by interaction with a neighboring galaxy. A Type Ia supernova on the southeastern side of this galaxy was discovered on Jan. 21, 2014. It is the closest Type Ia supernova spotted in 42 years.

Northeast of the star Megrez (Delta Ursae Majoris), there is an area seemingly empty of everything. There are no bright stars, infrared, ultraviolet or x-ray sources. This area is also in the Hubble Space Telescope's continuous viewing field, never blocked by the Earth as Hubble orbits it. Astronomers took 342 pictures of this area over a 10-day period in December 1995 with the Wide Field and Planetary Camera 2 through four different filters: near-ultraviolet, blue, red and near-infrared. These images were then combined into a color image that covered just 2.5 minutes-of-arc on each side.

Far from empty, this Hubble Deep Field (HDF) image contains 3,000 objects, almost all galaxies. These galaxies are all different distances from us, giving us an opportunity to study galaxies at many different ages. The most distant in the HDF are some 12 billion light-years away. The wide range of galactic ages allowed astronomers to make an estimate of the star formation rate in the universe. Star formation is estimated to have peaked between 8 billion and 10 billion years ago.

Also of interest was the lack of very faint nearby stars found in the HDF. Astronomers have been trying to determine where the excess mass is in a galaxy. For a number of reasons, they believe there is more mass in a galaxy than we can see; it was thought this mass might be many very faint red dwarf stars in the halo around a galaxy. Since these did not show up in the HDF, the excess mass, now referred to as dark matter, is not made up of red dwarfs. We still do not know what it is, but the search continues.

The Planets for May

Mercury pops out of the evening twilight just after the month begins. Among the constellations, it travels from Aries, all the way through Taurus, and into Gemini as the month ends. Mercury reaches its greatest distance from the Sun on May 25, when it will be magnitude +0.6 with a disc 8.3 seconds-of-arc across in the form of a crescent that is 35% illuminated. When Mercury first appears from the far side of the Sun, it will be almost full. As the month proceeds, it will become less gibbous, reaching the half-phase on May 19. From there, it continues into the crescent phase, becoming thinner every day. Mercury sets around 9:45 p.m. at the end of the month.

Jupiter is four months past opposition and can be found about 40 degrees up in the west as it gets dark, setting before midnight. At midmonth, it is magnitude -2.0 with a disc 33.9 seconds-of-arc across. The King of the Gods is moving slowly eastward in central Gemini.

The God of War is just past opposition and shines at magnitude -0.8. Its disc is 13.3 seconds-of-arc across at midmonth. Mars is moving westward in central Virgo at the beginning of the month. It reaches a stationary point on May 21, then turns back eastward and ends the month almost exactly where it started. This is a great time to take a telescopic look at this planet while it is still high in the evening sky, not setting until 4 a.m.

Saturn reaches opposition on May 10, making this a good time to observe the Ringed Planet as well. It is moving slowly westward in central Libra, visible all night. At opposition, it shines at magnitude +0.0. Saturn's rings are 42.2 seconds-of-arc across and tilted down 21.7 degrees with the northern face showing. Its disc is 18.6 seconds-of-arc across.

Venus rises around 4:15 a.m. in the east. Its disc is 15.2 seconds-of-arc across and is 72% illuminated, becoming fuller every day as it heads toward the far side of the Sun. The Goddess of Love starts the month in southern Pisces moving eastward. It cuts across a corner of Cetus and reenters Pisces. Venus ends the month just over the border in western Aries.

In 2008 comet 209P LINEAR was discovered here in New Mexico at LIncoln Near-Earth Asteroid Research center on the White Sands Missile Range. This month, 209P LINEAR will be coming back around, passing closest to us on May 29. The Earth will pass through the debris scattered along the orbit of this comet on May 24 between 1 a.m. and 2 a.m. Models suggest that we will intercept much material from the comet, so this may be a meteor storm with a radiant in Camelopardalis. So set your alarm to see this event and "keep watching the sky"!

An amateur astronomer for more than 40 years, Bert Stevens is co-director
of Desert Moon Observatory in Las Cruces.